The present invention relates to a matching cable which couples, with excellent matching, an automobile antenna for FM broadcasts and a receiver capable of receiving FM broadcasts.
Automobile antennas have been conventionally used widely and generally have an antenna element which is rod-shaped and telescoping, and a coaxial cable (to be referred to as a cable for brevity hereinafter), one end of which is connected to the feeding point of the antenna element and the other end of which is connected to the receiver. The antenna element is generally 1.0 to 1.2 m in length and is designed to have an electrostatic capacity (to be referred to as a capacity for brevity hereinafter) which, when the antenna element side is viewed from the other end of the cable during the reception of a medium frequency wave, is the capacity the receiver requires for the same medium frequency wave, for example, 80 pF.
Conventional automobile antennas are suitable for reception of amplitude-modulated medium frequency waves (to be referred to as AM waves for brevity hereinafter) and are not specifically designed in terms of impedance matching for receiving frequency-modulated VHF waves (to be referred to as FM waves for brevity hereinafter). For receiving the FM broadcasts, depending on the receiving band and the length of the antenna element, the voltage standing wave ratio (to be referred to as VSWR hereinafter) for 75 ohms measured at the other end of the cable is great, and it tends to become mismatched with the FM receiver. Such a condition is shown in FIG. 1. FIG. 1 is a graph showing the relation of the FM wave frequency and the VSWR of an automobile antenna consisting of an antenna element of 1.0 m length and a cable of 1.2 m length attached thereto, wherein the FM wave frequency is plotted along the abscissa and the VSWR (at 75 ohms) is plotted along the ordinate. As shown in FIG. 1, with this antenna, the VSWR is small in the FM broadcasting band (76 to 90 MHz) allowed to use in the region shown by FA of the figure, for example, in Japan, so that broadcasts in the FA region may be received well. However, for receiving the FM broadcasting band (88 to 108 MHz) allowed in the region shown by FB, for example, in the U.S.A. and European countries, the VSWR becomes greater so that problems arise such as energy losses due to mismatching of the antenna with the receiver, degradation in the receiving sensitivity, generation of distortion in the FM broadcasts and so on. Although the VSWR characteristic for an antenna element of 1.0 m is shown in FIG. 1, the VSWR characteristic becomes greater and can even reach 15 when the antenna element is longer. Attempts are recently being made in the U.S.A. to shorten the length of the antenna element to make it close to a quarter wavelength of the FM broadcasting band. However, it is well known that better sensitivity may be obtained with a longer antenna length when matched for FM broadcasting.